1. Field of the Invention
The present invention relates to a toner for developing electrostatic images in electrophotography, electrostatic recording, and electrostatic printing, as well as to an image forming apparatus and image forming method using the toner.
2. Description of the Related Art
It has been proposed so far that, in order to improve low temperature-fixing property of toner, polyester resins are used as binder resins for toner (see Japanese Patent Application Publication (JP-B) No. 05-82943 and Japanese Patent (JP-B) No. 3051767). However, in order further to improve the low temperature-fixing property of toner, it is necessary to reduce the average molecular weight and glass transition temperature of polyester resins, which causes a problem of reduced blocking resistance of toner under high-temperature and high-humidity conditions.
As a toner production method, a pulverization method is widely used in which additives such as a colorant, wax and so forth are added in a binder resin, the resultant mixture is heated, melted and kneaded, and the kneaded product thus obtained is cooled, pulverized and classified to produce toner particles with given particle diameters.
The pulverization method, however, has such problems as large variations between particle diameters of the produced toner particles, poor productivity, high costs and, particularly upon production of small-diameter toner particles, very low yields.
Consequently, in recent years, an emulsification polymerization aggregation method has been attracting attention as a toner production method in which the shape and particle size distribution of toner can be arbitrarily controlled (see JP-B No. 3246394 and Japanese Patent Application Laid-Open (JP-A) No. 2001-305797).
Furthermore, recently, speeding up and saving of energy is requested for image forming apparatuses, however conventional binder resins for toners cannot afford such marketing needs, and binder resins with further improved low temperature-fixing property are requested. When the softening point of polyester resin is reduced in order to achieve the low temperature-fixing property satisfying the marketing needs, toner particles tend to aggregate, resulting in poor storage stability. In addition such a toner often causes thermal fixing (filming) and so forth due to poor dispersion of releasing agent.
When a toner is produced by the emulsification polymerization aggregation method using the polyester resins, a dispersion of fine resin particles is prepared by emulsification polymerization. Meanwhile, a dispersion of fine colorant particles and a dispersion of wax as a releasing agent are prepared. A toner is produced by mixing these dispersions, adding a flocculant such as an inorganic metal salt to the resultant mixture with stirring to aggregate the fine resin particles and the fine colorant particles and so forth, and heating them for fusing.
However, the toner production method described above results in uneven dispersing of colorant and wax in the toner, causing them to be aggregated and exposed on the toner surface leading to wax spent and resin spent to carrier particles. In addition this production method has other problems that since the charge amount of the toner varies, the image density also varies and fogging occurs in the formed image.